<code>markAvailable</code> - marks all the available magnets or elements when a link is dragged (being reconnected). Default is <code>false</code>. This gives a hint to the user to what other elements/ports this link can be connected.
                                What magnets/cells are available is determined by the <a href="#dia.Paper.prototype.options.validateConnection"><code>validateConnection</code></a> function.
                                Internally, available magnets (SVG elements) are given the <code>'available-magnet'</code> class name and all the available cells the <code>'available-cell'</code> class name. This allows you to change the
                                styling of the highlight effect.
                